If your Pokemon who is neutral to water type Pokemon, you'd have to worry more about stray Hydro Pumps than you would have to worry about aqua tails. 4 EVs could be the difference between 1 HP and 2, and we are in a meta where even Physical sweepers are relying on lucky (low accuracy) Special Attacks with superior base power to the strongest alternative physical move.
It was established in 2007 in the F-6 study that 5 in 36 Pokemon Battles that go into the red on a stat with 0 EVs invested in that defensive stat would have otherwise gone into a KO. It was 17 in 39 with 4 EVs. I understand this is rare, but the evaluation had some other data. I don't remember the exact number, but I do distinctly remember that Special Attacks hit that substantial range somewhat more often than Physical during the event, and that Physical attacks were more prone to either overkill substantially or to fall into the 25% range.
I always favor Sp. Def for this reason when doing my last sliver of EVs. I have experienced a few cheap wins in Sp. Def territory, to my relief, specifically on an unfortunate RNG roll from my opponent getting the low end on an otherwise fatal strike.
Edit Also... Death Rays...
